The present invention relates to a servo system in a data storage device and, in particular, to the demodulation of position error signals (PES) within the servo system.
A data storage device, such as a magnetic disc drive, stores data on a recording medium. The recording medium is typically divided into a plurality of generally parallel data tracks. In a magnetic disc drive, the data tracks are arranged concentrically with one another, perpendicular to the disc radius. The data is stored and retrieved by a transducer or xe2x80x9cheadxe2x80x9d that is positioned over a desired data track by an actuator arm.
The actuator arm moves the head in a radial direction across the data tracks under the control of a closed-loop servo system based on servo data stored on the disc surface within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback servo signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system rotates the actuator arm to adjust the head""s position so that it moves toward a desired position.
There are several types of servo field patterns, such as a xe2x80x9cnull typexe2x80x9d servo pattern, a xe2x80x9csplit-burst amplitudexe2x80x9d servo pattern, and a xe2x80x9cphase typexe2x80x9d servo pattern. A null type servo pattern includes at least two fields, which are written at a known phase relation to one another. The first field is a xe2x80x9cphasexe2x80x9d or xe2x80x9csyncxe2x80x9d field, which is used to lock the phase and frequency of the read channel to the phase and frequency of the readback signal. The second field is a position error field, which is used to identify the location of the head with respect to the track centerline.
As the head passes over the position error field, the amplitude and phase of the readback signal indicates the magnitude and direction of the head offset with respect to the track centerline. The position error field has a null-type magnetization pattern such that when the head is directly straddling the track centerline, the amplitude of the readback signal is ideally zero. As the head moves away from the desired track centerline, the amplitude of the readback signal increases. When the head is halfway between the desired track centerline and the centerline of the adjacent track, the readback signal has maximum amplitude. The magnetization pattern on one side of the centerline is written 180xc2x0 out of phase with the magnetization pattern on the other side of the centerline. Thus, the phase of the readback signal indicates the direction of the head position error.
To control the servo system, a single position error value must be determined for each pass over the position error field. Typically, the magnitude of the position error value indicates the distance of the head from the track centerline, and the sign of the position error value indicates the direction of the head""s displacement. Demodulating the readback signal associated with the position error field typically creates the position error values.
Demodulation of the readback signal from the null pattern has, in the past, always been a synchronous process. In a synchronous process, the exact phase of the readback signal from the position error field is known from the phase field""s readback signal because the phase field is written on the storage medium at a known and fixed phase relation to the position error field. A phase-locked loop (PLL) is typically used to acquire the phase of the phase field, and this phase information is used for demodulating the position error field. The phase field must therefore be sufficiently long to enable the PLL to lock on to the phase and frequency of the readback signal. For example, the phase field may be 3 times longer than the position error field.
In a servo sector scheme, with servo fields interleaved with data fields, long phase fields consume valuable data sectors on the storage medium. These data sectors could otherwise be used for storing data. As disc storage capacity requirements continue to increase, there is a continuing need for reducing the area consumed by servo data.
The present invention addresses these and other problems, and offers other advantages over the prior art.
The present invention relates to an asynchronous analog demodulator and method, which solve the above-mentioned problems.
One embodiment of the present invention provides a method for determining a position error of a read head relative to a position on a medium in a storage device. The method includes steps of: (a) generating a read signal as the read head passes over a servo area on the medium; (b) generating a normal demodulating signal that is asynchronous with the read signal; (c) generating a quadrate demodulating signal that is ninety degrees out of phase with the normal demodulating signal; (d) multiplying the normal demodulating signal by the read signal to produce a normal position signal (e) multiplying the quadrature demodulating signal by the read signal to produce a quadrature position signal; and (f) producing a position error magnitude and a position error direction based on the normal and quadrature position signals.
Yet another aspect of the present invention provides a method for determining a position error value having a magnitude and a sign indicative of the distance and direction that a read head is displaced relative to a location on a storage medium. The method includes steps of: (a) generating a phase field read signal from a phase field on the medium; (b) generating a position error field read signal from a position error field on the medium; (c) demodulating the position error field read signal using at least one demodulating signal to produce at least one position error field coefficient, the at least one demodulating signal being asynchronous to the position error field read signal; (d) demodulating the phase read signal using at least one demodulating signal to produce at least one phase field coefficient; (e) determining the magnitude of the position error value based at least in part on the at least one position error field coefficient; and (f) determining the sign of the position error value based at least in part on the at least one position error field coefficient and the at least one phase field coefficient.
Another aspect of the present invention provides a disc drive storage device for accessing data on a storage medium. The disc drive includes a read head for generating a read signal. A servo system positions the read head over the medium based in part on a position error value that represents the distance and direction that the read head is displaced from a location on the medium. A normal signal generator generates a normal demodulating signal. A quadrature signal generator generates a quadrature-demodulating signal that is orthogonal to the normal demodulating signal. A normal multiplier multiplies the digital read signal by the normal demodulating signal to produce a normal position signal. A quadrature multiplier multiplies the read signal by the quadrature-demodulating signal to produce quadrature position signal. A magnitude determination circuit determines the magnitude of the position error value based at least in part on the normal position signal and the quadrature position signal. A sign determination circuit determines a sign of the position error value based at least in part on the normal position signal.
Yet another aspect of the present invention provides a disc drive storage device for accessing data on a medium, wherein the device includes a servo structure for positioning a head over the medium based on a position error for the head relative to the medium. The device further includes analog demodulation means for receiving a read signal from the head and generating the position error asynchronously to the read signal.